Experimental Study On Sulfoaluminate Cement Production With Semi-Dry FGD By-Products

In China,the coal-based energy structure leads to the case that thermal power generation is the main power form,and it is also the chief reason of excessive discharge of sulfur dioxide.At present,the main desulfurization technologies can be classified as desulfurization before combustion,during combustion,and after combustion(i.e.flue gas desulfurization).Generally speaking,based on the different forms of desulfurizers and by-products,flue gas desulfurization(FGD) technologies can be classified into wet FGD process,dry FGD process and semi-dry FGD process. Therein,the employment of semi-dry FGD process is just being accelerated,because of its features of simple process,low investment,less area requirement and no corrosion,etc.,However,the comprehensive utilization of semi-dry FGD by-products is a big issue.These desulfurization residues are featured as complicated composition and high contents of sulfur and calcium,which seriously limit their application in some troditional fields,e.g.using as building materials.About the disposal and utilization of semi-dry FGD residues,different methods, such as using them as baked brick materials or cement retarder,had ever been tried in our laboratory,but no promising results were obtained.Then,based on the fact that desulfurization residues and sulfoaluminate cement have the same elemental compositions,a new utilization method to produce sulfoaluminate cement was put forward.Subsquently,a series of experimental studies were carried out.At first,some groping experiments were carried out with mixed desulfurization residues being calcined in a high-temperature resistance furnace.Two hydraulic minerals,dicalcium silicate and calcium sulfoaluminate were obtained in the product samples.Then,actural desulfurization by-products obtained from semi-dry FGD process were used as materials and calcined in the high-temperature resistance furnace and a high-temperature rotary furnace respectively.And the effects of different feed proportions,calcination temperatures and calcination atmospheres on clinker burning,compositions and strengthes were tested.It was proved that at about 1300℃,in appropriate feed proportion,desulfurization residues with fly ash and limestone could be converted into sulfoaluminate cement which took dicalcium silicate and calcium sulfoaluminate as main minerals,and the content of calcium sulfoaluminate could be increased by adding some high-alumina minerals into minerals.As dicalcium silicate guarantees later strength of clinker,and calcium sulfoaluminate has good early strength,the clinkers obtained from the experiments showed excellent mechanical strength performance.In order to implement this feasible technology in industrial production,a pilot test was carried out in a cement rotary kiln.There was no need to change original equipments or add new equipments, and only by altering material,desulfurization residues could be converted into sulfoaluminate cement continuously,moreover,cost and energy consumption during production were reduced dramatically.The clinkers obtained from pilot production took dicalcium silicate and calcium sulfoaluminate as main minerals,and satisfied the strength requirement of building industry.In view of the possibility that instability of sulfur-containing may result in emission of SO₂,intensive experimental research was carried out to determine the migration characteristics of sulfur.In the experiments,the thermal stability of calcium sulfite and calcium sulfate in desulfurization residues were studied under different calcination atmospheres.It was proved that,under aerobic environment,calcium sulfite in the desulfurization residues could be oxidized into calcium sulfate entirely at about 700℃.Since in cement production lines,raw materials are heated over 800℃in cyclone preheaters,all the CaSO₃ in FGD residues could be oxidized into CaSO₄ in this stage,then all CaSO₄ in the materials can be stiffened in calcium sulfoaluminate by high-temperature solid phase reaction.Moreover,calcium sulfoaluminate has high-temperature stability and doesnot decompose below 1400℃.As stated above, SO₂ wonnot escape in the convertion of desulfurization residues into sulfoaluminate cement,therefore,it is completely feasible to implement this project.The implemention of this technology could bring about remarkable economical benefits to both electric enterprise and cement enterprise.Firstly,an electrical enterprise could benefit from selling FGD residues.Secondly,a cement enterprise could reduce production costs by reducing material costs,grinding power consumptions and calcination coal consumptions.Using desulfurization residues to produce sulfoaluminate cement,could not only save energy,but also solve the land occupation problem of solid waste.Finally,the technology could solve subsequent problem of semi-dry FGD technology,and promote the popularization and application of semi-dry FGD technology indirectly.